Clarify supercompression for UASTC formats

[lexaknyazev]

The current spec says:

UASTC images can be supercompressed with Zstandard (supercompressionScheme = 2) with or without first conditioning the data with Rate-distortion Optimization.

Similar language is being added in #216 (note that RDO is not mentioned for some reason):

UASTC HDR images can be supercompressed with Zstandard (supercompressionScheme = 2).

Both of those statements are not helpful because technically any KTX 2.0 file (except those using BasisLZ) can be supercompressed with Zstandard.

The real question is: are UASTC formats limited to Zstandard, i.e., do they disallow use of Deflate? If yes, the spec should be explicit about that; if no, the quoted language should be more generic.